Sound horns currently equipping motor vehicles make use of the vibration of a membrane under the effect of an electromagnetic system. However, these electromagnetic sound horns, which are used on a world-wide scale, do have the drawback of being heavy (about 250 g) and consume a large amount of electricity (between 4 and 8 amps under a serviceable voltage of 12 V).
The aim of the present invention is to mitigate the drawbacks mentioned above of electromagnetic sound horns and to this effect concerns the embodiment of a piezoelectric horn, that is a sound horn using the vibration of a piezoelectric membrane.
This effect known as the &lt;&lt;piezoelectric effect &gt;&gt; is already known by which electrical charges appearing on the faces of certain crystals (quartz in particular) when the latter are subjected to mechanical stresses. As these crystals are generally nonconducting, the charges appearing on their faces give rise to electric voltages, possibly extremely high, and these electric voltages are picked up by metallic electrodes on the faces of the crystals.
This piezoelectric effect is reversible. If an electric voltage is applied it, this results in this crystal being mechanically deformed.
The piezoelectric effect is characteristic of the order structures of the material (crystals), but mono-crystaline materials are rare and scarcely suitable in use. However, certain ceramics have a crystalline type structure lending themselves also to the use of piezoelectric effects in consideration of a prior orientation of the molecules constituting the ceramic material. This orientation is obtained by applying for several seconds a high electric polarization field on the ceramic material, thus provoking the alignment of the molecules inside the ceramic material. On removal of the electric polarization field, the molecular orientation exists and gives rise to a piezoelectric phenomenon identical to the one observed in the monocrystals.
The polarized ceramics are much more suitable in use than the crystals and in fact are obtained in the desired shape by means of sintering at a high temperature and baking at more than 1000.degree. C.
So as to produce sounds with the aid of a piezoelectric ceramic material, it has already been suggested to make use of a piezoelectric sound membrane formed of a thin metallic disk on which a piezoelectric ceramic disk is glued. A counter-electrode constituted by conductive ink is laid on the free surface of the ceramic disk and polarization is effected so that, when an alternative voltage is applied to the ceramic disk, the latter dilates or contracts diametrically. As these diameter modifications are set on one of its faces by the metallic membrane, everything is bent inward in the way of a bimetallic strip alternately in one direction and then in another, thus producing a vibration corresponding to the frequency of the excitation voltage.
So as to effectively use the piezoelectric membrane, the latter is mounted in a suitable support, this mounting device taking a significant part on the sound emission of the membrane in modifying the resonance frequencies of the membrane and more generally the frequency response of the device.
It is also known that this frequency response is likely to be modified by the use of flat non-circular metallic structures (especially rectangles) used instead of the thin metallic disk or in conjunction with the latter.
The piezoelectric membranes currently available on the market are solely used for low sound level applications. These applications mainly concern telephony, low level sound reproduction on portable devices, and also buzzers for industrial equipment and instrumentation, in particular motor vehicle buzzers (warning of neglecting to turn off the headlights, one door having remained open or an engine defect). The only applications with a sound level of more than 100 dbA concern either high frequency generators (1800 to 3600 Hz for car horns) or applications with a narrow frequency band and limited power (105 dB to 1000 Hz for reversing horns for lorries and engines).
These known types of low sound level buzzers of about between 70 and 90 dBA in a free field 10 cm away use small diameter piezoelectric membranes (generally less than 50 cm) having a resonance frequency of between 2 and 3 Khz.
This known conception of piezoelectric membrane buzzers, owing to its slight sound level, are unsuitable for the embodiment of sound horns for motor vehicles whose acoustic characteristics, established by the European Directive 70/388/CEE and by the recognized international agreement known under the designation &lt;&lt;Rule 28&gt;&gt;, need to be clearly much high-performing.
In fact, the sound level measured 2m away from the horn in a free field (on in a dead room) need to be between 105 dBA and 118 dBA and the sound level of this horn, once mounted on the vehicle, needs to be 93 dBA to 7 meters at the front of the vehicle.
Having regard to sound isolation, which increases in the vehicle engine compartment, this latter condition requires that the horn be adjusted to be as close as possible to the authorized maximum of 118 dBA, which has no relation with the 90 dBA measured 10 cm away obtained with the buzzers mentioned earlier which correspond to about 65 dBA measured 2 m away.
In addition, although this is not formally laid down, usage for sound horns consists of using frequencies of between 300 and 500 Hz which renders the sound devices used in the horns as being inadequate for a horn application.